1. Field of the Invention
The present invention relates to a fuel injection system for a two stroke internal combustion engine. The present invention may be also applied for a two stroke engine having 4 valves wherein a large amount of air is blown out due to a long valve overlap period in which both the intake and exhaust valves are open.
2. Description of the Related Art
In a two stroke engine, a long period occurs in one engine cycle wherein both intake and exhaust ports are open. During this long period, a large amount of fuel is blown out to the exhaust pipe before being combusted in the cylinder bore, when the fuel is introduced into the cylinder bore in the form of a combustible mixture from a carburetor in the usual way, and this blowing out of fuel reduces the fuel consumption efficiency. Therefore, a system was proposed wherein a fuel injector supplies a calculated amount of fuel to the engine at a predetermined period so that the fuel will be effectively combusted in the cylinder bore. The blowing of air to the exhaust pipe before combustion is, however, inevitable regardless of the fuel injection system used. Namely, only a part of the total amount of air introduced into the cylinder bore is actually combusted, and therefore, the amount of fuel to be injected from the injectors is calculated from an amount of air as sensed, multiplied an air-capturing ratio which corresponds to the ratio of the amount of air captured in the cylinder bore for combustion to the total amount of air, so that the combustible mixture in the cylinder bore actually combusted becomes equal to a predetermined air fuel ratio required for the combustion operation. This captured air ratio, however, has different values, which are obtained in accordance with the engine basic operating conditions including an engine load and speed.
To obtain a desired air fuel-ratio value regardless of whether or not the captured air ratio is varied in accordance with engine load and speed, an improved system has been proposed wherein the basic fuel amount as calculated is corrected in accordance with the captured air ratio, which is calculated in accordance with an engine load and speed. See Japanese Unexamined Patent Publication No. 53-27731. In this prior art, the correction factor is calculated in accordance with an engine load and speed by using an algebraic function, such as an exponential function. In other words, a change of value in the captured air ratio is approximated to an exponential function, and this function is used to determine the captured air ratio for calculating the corrected amount of fuel to be injected.
This prior art is based on the concept that the captured air ratio changes in accordance with engine operating conditions along an exponential function. However, the actual captured air ratio in a two stroke engine has a complicated characteristic, since it is not only affected by the basic factors, i.e., engine load and speed, as usual, but also affected by a particular factor in a two cycle engine, i.e., a blow down of the exhaust gas. Therefore, a precise correction of the blown out air by a mere approximation using an exponential function is very difficult, and thus a precise control of air fuel ratio in accordance with the captured air ratio cannot be realized. Accordingly, it becomes impossible to obtain a desired air-fuel ratio, thus lowering the fuel consumption efficiency and causing an excessive increase in the temperature of the catalytic converter.